Carmel's weather profile is close to ideal for marathon performance, and your clothing choices should reflect that. The goal is to be as light and aerodynamic as possible while handling the 15 to 20 degree temperature swing from the 8:10 AM start to your finish time.
Typical conditions (start 38 to 45°F, finish 52 to 60°F). This is the most common scenario and the easiest to dress for. Short-sleeve tech shirt or singlet, running shorts, and light gloves for the start that you can tuck into your waistband by mile 3. A throwaway long-sleeve layer for the corral is smart if you run cold. You should feel slightly chilly at the gun and comfortable by mile 2. If you feel warm at the start, you're overdressed.
Cool day (start below 38°F). Occasionally, mid-April in Indiana delivers a genuinely cold morning. Light long-sleeve base layer, shorts, light gloves, and a headband or thin cap for the ears. Arm sleeves are a good alternative to a full long-sleeve if you want the option to push them down as the day warms. You'll warm up on the course, so resist the urge to add more layers. A 35°F start with a marathon effort becomes a 55°F feel within 15 minutes.
Warm day (start above 50°F, high above 65°F). This changes the race. Singlet or lightest tech shirt possible. Short shorts. Light-colored hat or visor. Sunglasses. Sunscreen, especially for the exposed road sections in the second half. On a warm day at Carmel, the second-half uphill trend becomes meaningfully harder because heat compounds the fatigue.
Rainy day. Carmel has been run in heavy rain. Moisture-wicking everything, no cotton. A light cap with a brim to keep rain out of your eyes. Anti-chafe applied aggressively to all friction points (nipples, inner thighs, underarms, feet). Do not wear a rain jacket for 26.2 miles. You will overheat. Getting wet is fine. Overheating is not.
Wind. Indiana is flat and open. On a windy day, exposed straightaways like Westfield Boulevard can feel like running into a wall. You can't dress for wind in the same way you dress for cold, because the effort of running generates enough heat that wind doesn't make you cold, it just slows you down. The best "gear" for wind at Carmel is tactical: run behind other runners or your pace group and let them break the wind.
Shoes. The course is road and paved trail. Standard road racing shoes are the right call. If you're chasing a BQ and you have a plated racing shoe you've trained in, this is the course to wear it. The terrain is forgiving enough that you don't need extra cushioning, and the flat profile lets you take full advantage of whatever energy return your shoes offer.
The short version. Most years: singlet or light tech shirt, shorts, light gloves, and nothing else. Carmel is a race where speed matters and every ounce counts. Dress for performance, not comfort. If you're comfortable at the start line, strip a layer.